Photoelectric device for indicating the diminution of light



F LIGHT E. MElLl Jan. 1, 1952 PHOTOELECTRIC DEVICE FOR INDICATING THE DIMINUTION 0 Filed Sept. 9, 1947 Patented Jan. 1, 1952 PHOTOELEOTR-IC DEVI THE DIMINUTI CE FOR INDICATIN G ON OF LIGHT Ernst Meili, Bad Ragaz, Switzerland, assignor to Elektro-Watt Elektrische und Industrielle Unternehmungen Application September 9 In Switzerland 14 Claims.

The present invention relates to a device indicating modifications in light for operating light relays or other electrical release apparatus.

The device utilizes a gas discharge relay with at least three electrodes, at least one of which is light-sensitive. This device is operated by suitable auxiliary devices which produce an are between the light-sensitive electrode and another electrode, said are being influenced by changes in light. Thus in the case of the present invention a physical process is used Which occurs at the transition from a non-spontaneous to a spontaneous discharge.

In order to make this operation easier to comprehend, it is first necessary briefly to elucidate the physical processes of discharge by reference to Figs. 1 and 2. In these and subsequent figures corresponding parts are designated as far as possible with the same letters. It is assumed that in a sealed vessel G filled with a gas at low pressure there are two electrodes Z and L, one of which, L, is provided with a photo-sensitive coating P. Here, as in the subsequent text, light is to be understood as meaning not only visible but also infra-red or ultra-violet light. With appropriate special arrangement gas filling such discharge gaps show in the dark a fallin voltage characteristic from the point of ignition onwards. In Fig. 2 curve a represents such a characteristic. The current i is the abscissa and the electrode voltage U is the ordinate. The discharge voltage U of the spontaneous discharge is called the ignition voltage. According to the choice of the discharge parameters (type of gas, gas pressure, form of and interval between the electrodes) and of the external circuit elements, (resistance, capacity, inductivity, etc.) the discharge burns at low currents, i. e. within the range of the so-called Townsend discharge, either stably or unstably. If, for example, the discharge gap ZL in Fig. 1 is connected via a high-ohmic resistor R to a source of current V and a small capacitor C is connected in parallel to the discharge gap, no current flows when the electrodes are not exposed so long as the voltage V is lower than the ignition voltage Uo. If, on the other hand, the voltage V exceeds the ignition voltage U0, a gas discharge ignites in the tube, and a spontaneous gas discharge current i fiows which in the case of stability assumes a certain value determined by voltage and resistance, but which in the case of instability rapidly reaches higher values, the voltage U at the same time breaking down at the electrodes.

During this discharge process the current i is in the main supplied through the capacitor C.

A. G., Zurich, Switzerland, a company of Switzerland 1947, Serial No. 773,036

January 18, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires January 18, 1966 The voltage collapse of the capacitor C may reach as low a value as that of the burning voltage of the glow discharge or even lower. As a rule the discharge then breaks oif, the electrodes re-' charge to ignition voltage, whereupon the process is repeated. This tipping is designated below as surge discharge. These surge discharges can also occur when no special capacitor is connected in parallel, in that only the self-capacitance of the electrodesand the capacitance of the supply conduits operate as discharge capacitor. The surge discharges are, of course, then correspondingly weaker.

According to the invention an arrangement is used for the device described below in which the Townsend discharge is unstable in the falling section of the current-voltage characteristic in the sense of the above description. Many gas discharge gaps display a similar behaviour, it is true, but very special conditions have to be complied with for this tipping or relaxation process-to take place even with minimum currents (less than 10- A.) and minimum capacitances (a few pf.). Unless special precautions are taken, gas discharge gaps show with very low currents, as a rule, either small predischarges without an actual surge discharge with considerable voltage collapse occurring, or the Townsend discharge burns stably (with either a rising Or a falling characteristic) so that here again no surge discharges take place.

If the cathode of a discharge gap as described above is irradiated with light, electrons are released from the cathode which produce a primary photo-current it. As the voltage U increases an increase in the strength of this photo-current in by gas ionisation takes place, an effect which is utilized in the ordinary gas-filled photo-electric cells. If the voltage U is stepped up still further, the current increases until a certain point Pz is reached from which onwards increasing current corresponds to decreasing voltage. The value of the current 2' and the voltage U corresponding to the point Pz will hereinafter be designated as ignition current iz or with irradiation ignition voltage Uz. In Fig. 2 the curve In represents the course of the current-voltage characteristic for a relatively low intensity of irradiation Bl.

Theoretical experimental investigations have revealed that, with an increasing intensity of irradiation, the ignition current iz increases and the ignition voltage Uz decreases. In Fig. 2 the curve 192 accordingly represents the course of the characteristic for a higher intensity of irradiation B2. It is again of decisive importance for the process according to the invention that the discharge is unstable on the falling portion of or relaxation process with irradiation, that is to been passed the characteristic say after the ignition point Pz has a surge discharge occurs.

The process used in the device according to the invention consists (of. Fig. 2) in that at a definite intensity of irradiation B2 a definite preliminary current is is caused to pass across the discharge gap which is lower than the ignition current in, i. e. the working point P2 appertaimng to 7:9. lies on the rising and therefore stable branch of the characteristic. If the irradiation is reduced the characteristic is displaced in such a Way that a lower ignition current corresponds to the point of ignition. With adequate dimming of the light, for example to the value B1 (characteristic b1), the preliminary current is exceeds the ignition current in; the theoretical working point P1 lies on the falling branch of the characteristic in and a surge discharge occurs. This surge discharge recurs at periodic intervals, the intervals being deter mined by the charging current and the discharge capacity. As soon as irradiation again attains theoriginal value of B2, the surge discharges cease.

Theoretical and experimental investigations have revealed. that with level, parallel electrodes within. the range of weak irr'adiations the igni tion current iz is proportional to the root of the current in photo-electrically released at the cathode, i. e. the ratio is (K=proportlonality constant). It is evident from this that the ratio of ignition current z'z to primary photo-current io assumes the value therefore, the lower the primary current is, the greater this ratio becomes. As, with appropriate designing: of the discharge gap, K assumes values of the magnitude of 1-400", this means that with low intensities\' of irradiation enormous increases in the photo-current are possible up tothe point of transition to spontaneous discharge. This ratio may easily amount to one million and over. Provided the total quantities of light remains constant the ratio becomes greater as a rule, if large-surfaced electrodes and small. g'apsar'e used and the light is ditributed over a large area.

In the above description a Townsend characteristic falling linearly from the point of ignition onwards was pre-supposed. The effect is in no way restricted to this case, however, but it is sufficient if the characteristic is in any way falling; it is in. certain cases even admissible for the characteristic to rise first and only to fall from a certain point. 0n the other hand, it is essential in every case that the ignition current should increase with increasing irradiation and. that after the point of ignition has been passed a surge discharge should occur.

The use of gas-filled photo-electric cells and arrangements in which glow gaps are controlled by lightis known.

Such ordinary gas-filled photo-electric cells are connected by means of a high resistance or other current-limiting device to a voltage which is higher than the ignition voltage. Then a suitable selection of elements will produce so called. tipping oscillations, the frequency of which is determined by illumination and which may be used for the further operation of auxiliary devices. Since the currents are small, a separate amplifier is usually necessary.

The difference between the present invention and devices in which the principle of ignition voltage decrease by irradiation is applied is that, according to the invention, no tipping frequency is produced or influenced, but only the fact is made use of that with increasing irradiation. the tipping is prevented and with sufficient diminution of the strength of illumination a current discharge is made possible. An important feature of the invention is that the light-sensitive discharge gap is used as an ignition gap of a multiple-electrode gas discharge relay, as will be described in detail hereinafter.

Instructions are given below for the practical application of the process:

The photo-sensitive coating or layer may be applied either direct to the vessel wall or it may be used to coat a special electrode. For certain uses it may be expedient if the photosensitive layer is transparent so that exposure can be effected from behind. The photo-sensitive coating may, in principle, take the form of any coating such as is used. for photo-electric cells, especially coatings of alkali and alkaline earth metals or combinations of these. For special requirements highly sensitive so-calledsynthetic coatings or so-called alloy cathodes" are best used. As according to the invention an enormous amplification of the primary photo current can be achieved, it is frequently not necessary at all to have a highly sensitive coating. In many cases more importance will be attached to other properties, such as great temporal constancy, insensitiveness of the layer to the gas discharge and temperature resistance. These conditions are, as a rule, much easier to achieve with simple coating such as potassium or barium coatings.

The ignition-electrode must be so arranged in relation to the photo-electrode that even with the weakest currents (10 A.) and minimum capacitance (a few pf.) a discharge, unstable in the falling section of the characteristic, occurs, which should be as powerful as possible. This behaviour is favoured by a great difference between ignition voltage and burning voltage and by a small. cross-sectioned area of discharge. These conditions may for example be complied with. by giving the anode small dimensions in relation to thecathode and keeping the intervening distance small. (Ignition of a tip or of a thin wire against a surface.) The tipping or relaxation. process may furthermore be promoted by the provision of insulating or conductive partsbetween the ignition electrode and the photo cathode, through the presence or field influence of which the ignition process is influenced.

Insofar as the ignition electrode is near the photo-electrode, it is only necessary to exposethe photo-cathode lying directly.

that part of opposite the ignition electrode. In such a case it is expedient to concentrate the incident rays on this spot through a condenser lens or a con cave mirror. In this way a high degree of light intensity is achieved even with relatively weak irradiation and in addition to this a marked degree of directional sensitivity of the arrange-t ment is achieved.

The filling gasis of importance for the roper operation of the tube. Hydrogen has proved very suitable, but as a rule rare gases are preferred as these generally produce lower ignition voltages and act less on the electrodes by chemical reaction, adsorption and diffusion. The filling gas. may consist of one single gas or of mixtures of at least two rare gases, possibly with the addition of common gases. It has proved to be of advantage to select a relatively high gas pressure, namely, according to the type of gas, up to and even over 100 tor. It is possible by suitable mixtures and pressures to render the ratio particularly high.

As already mentioned, it is necessary to induce a certain preliminary current through the gap. This current, which is very low according to circumstances and may be of less than A., may be'produced in diiferent ways.

An expedient arrangementconsists in connecting a suitable reactance to a driving voltage which is considerably greater than the ignition voltage of the gap, e. g. by the pre-insertion of resistor, an ionisation chamber'with radio-active radiation, a glow tube, a pre-exposed photo-electric cell, a suitable gas discharge path (possibly with a photo-sensitive cathode). The ionisation chamber, glow tube and photo-electric cell present the advantage that within certain voltage ranges the current is practically independent of the voltage, thus providing very constant conditions. Glowing cathode and photo-electric cells also have the additional advantage, that, operated with alternating current, they rectify the curre nt. If the cell is operated with alternating current or temporallyinconstant direct current, a capacitor may also be pre-inserted. In order to adapt the preliminary current to the intensity of exposure, it is in many cases indicated to provide a variable or exchangeable current-determining element.

'As, in accordance with the above remarks, the preliminary currents may, according to circumstances, be very weak, this means that one of the" feeders to the tube must be very well insulated. As highly-insulated places usually lose their highly insulating properties in time if exposed to the air, it is e'xpedient'to design the tube so that the insulated place does not come into contact with the outer air. This is achieved by building the current-determining element into or onto the tube. If a special surge capacitor is provided, it is advisable to arrange this in such a manner that its highly-insulated conduits do not come into contact with the outer air, for example, by placing the condenser within the tube, or placing it upon the glass walls in the form of conducting layers. It is moreover not necessary for this capacitor to be inserted directly between the electrodes; it may lie between one of the electrodes and any desired point which has a certain potential in relation to the counterelectrode. The discharge container may be connected with a second vacuum-tight container which contains the reactance and/or the condenser, or the terminals with the reactance may be embedded in wax or the like.

In Figs. 3-7 various embodiments are shown which. serve to explain the principle mentioned. In Fig. 3 both the reactance R and a possible parallel capacitor C are housed inside the discharge vessel G. In Fig. 4 this reactance consists of an ionisation chamber R provided with a radio-active preparation R-s,'-the ionisation emplified in Fig. 6 and consists in that the reactor R e. g. a resistor, is entirely embedded in aninsulating material which at the same time encloses the feeder to the cell so that it does not come into contact with the outer air.

Since with the arrangements described the surge discharge takes place entirely within, it is, if no special measures are taken only possible by optical means to observe the tipping or relaxation process. In order to be able to observe this process by electrical means a capacitor can beprovided, without leading the highly insulated electrode conductively to the outside, one electrode of which capacitor is located outside thevessel so that the surge discharge can be observed or used to control a further contrivance outside the cell. In Fig. 5 this is intimated by the capacitor C1 via which, for example, an ele eter or an amplifier can be controlled.

In the embodiment hitherto mentioned it was usually pre-supposed that the photo-electrodewas connected to the negativepole and the counter-electrode to the positive pole of the source of current. It will, however, be desirable in many cases to be able to operate the device with alternating current. It has already been mentioned that as current-determining element a reactor having a rectifying action may be used.

In this case operation with alternating current is easily possible; it is true that a discharge of the capacitor occurs through the photo-electric current during the period of blocking. The time constant of the device determined by current and capacitance must therefore be such that when the light is dimmed ignition ertheless take place. If no rectification takes place in the reactor R, it must be taken into account that in one half-wave the photo-electrodeis positive and the counter-electrode negative.

Either the tube can be designed in such a way and the driving voltage be selected so low that in this phase no ignition takes place, or the tube and voltage may be such that in the blocked phase ignition occurs without there being any surge discharge. Both electrodes may also be made photo-sensitive so that in both half-periods surge discharges occur or do not occur according to the intensity of irradiation. Finally it is also permissible for surge voltages always to-ocour in the one half-period (photo-electrode positive), whilst in the other half-period the surge discharges are controlled by light. Inall the cases mentioned the time constant must be so low that ignition can take place.

Since with technical alternating current'this generally requires relatively heavy preliminary currents, exposure must be correspondingly intensive. It may therefore be of advantage if a slower alternating current is selected as operating frequency. For certain requirements it may also be desirable to operate the device with some temporally variable direct or alternating current for instance with pulsating direct current or with a tipping" or relaxation-oscillation curctromcharging liminary current a capacitor C is used in the formof conductive linings on both sides of the insulating vessel-wall. With an increase in the voltage V the voltage at Z increases, so that upon exposure a preliminary current flows which provokes a certain degree of discharge of the capacitor and prevents ignition. With a decrease in irradiation conditions of ignition can, or the other hand, be fulfilled. I

'It is known in the art to ignite a gap the voltage of which is smaller than the ignition voltage, but greater than the operating voltage, by utilizing a comparatively weak discharge of a separate electrode. vention this discharge is introduced into the described tipping discharge of the light-sensitive apparatus. V

Fig. 8 shows an example of how this can be achieved. In addition to the ignition electrode Z and the photo-electrode L a further electrode A is provided which will here be called the intercepting electrode. The contrivance functions in such a way that the surge discharge occurring between Z and L leads to ignition between A and L also, i. e. the surge discharge is to a certain extent intercepted by the electrode A and maintained in the form of a secondary glow discharge between A and L. In the outer circuit a powerful current determined by voltage and resistance can therefore flow, which current operates the relay E. Since this discharge flows between the intercepting electrodes and the photo-electrode, destruction of the photo-sensitive coating may occur through bombardment by ions, especially if it is a synthetic coating. In order to reduce this effect, the cathode may, for example, be designed in such a way that the secondary glow discharge impinges chiefly on parts of the cathode which do not need to be photo-sensitive, which can, for example, be achieved by suitable spacing. An-- other solution is exemplified in Fig. 9. The cathode is subdivided in such a way that the part L which must be photo-sensitive is electrically separated from, and connected via a high-ohmic re-- sistor W to, the other part current can attain no considerable proportions on the part L of the cathode thanks to the said high-ohmic resistor.

Fig. 10 shows how this separation can be achieved with only three electrodes. The reactor R is inserted in the feed line to the photo-sensitive electrode L and the surge discharge, which ignites between this electrode and the electrode A, is intercepted between the electrodes A and Ii.

Fig. 11 shows an arrangement similar to that in Fig. 8 except that the ignition electrode and the intercepting electrode are arranged ondiflierent sides of the photo-electrode. 'The photo-electrode must in this case be perforated in the form of a sieve or mesh so that the surge discharge can extend also to the side .of the intercepting electrode.

The current load capacity of the electrodes is subject-tocertain limits; in order nevertheless In accordance with the present in- K, so that the glow glow discharge. It is to have sufficient control energy available in the secondary circuit it operating voltage which is than the burning voltage. voltage between the intercepting electrodes must be su-ificiently large. An increase in the ignition voltage may be achieved in the known manner by the following measures:

considerably higher (1-) By increasing the distance between the electrodes;

(-2) By increasing suitable gas;

(3) By the introduction of a further electrode which brings about at least a partial electrical the gas pressure choosing a screening of the principal electrodes, this elec trode either being connected to an auxiliary po-' tential or being 'in'sulatedly mounted;

(,4) By insulating bodies field configuration or prevent the formation or a discharge. An arrangement of this kind is suggested in Fig. 9, where the intercepting electrode A is concentrically surrounded by a small glass u e D;

(5) By suitable design of the electrodes e. g:

large-surfaced, cathode, small-surfaced anode (tip).

There is the possibility of operating not only the surge discharge gap but also the control .organ with alternating current or otherwise a tern pora'lly variable current.

glow discharge, once ignited, extinguishes again as the ray of light is released once more. eration with direct current this is notimmedi ately possible; although it can also be achieved. by the use of "tipping switching devices which bring about the interruption of the glow discharge, for example, by the insertion of a relatively large capac'ty and a suitably large load reista-nce in the main circuit. I

The number of switching or circuit elements required with alternating current is particularly small if rectifying action is present in the main insert a direct current relay in the outer circuit, possibly with a capacitor connected in parv known rectifying effect due to greatly differing surface areas Of the anode and cathode, or to difierent ignition voltages on the changing of poles (glow rectifier) the primary surge discharge may also efiect rectification either by the surge discharge only occurallel. Apart from the ring in the .one phase or by the spatial arrange. ment being such that a surge other phase causes no ignition between the intercepting electrodes. This occurs,

trode and opposite sides of a mesh-like photo-cathode.

I claim:

1. An electric device for indicating changes in intensity of irradiation, comprising a gas-filled tube comprising an airtight casing, a first photo sensitive electrode and said second electrode and. passing a weak e1ec-.I

including a reactance for trical preliminary current through said control 1 gap, the magnitude of said preliminary current is of advantage to use an For this the ignition which influence the Apart from saving the rectifier this may present the advantage that a then merely necessary to discharge in the for example, with an arrangement wherein the receiving elec-. the ignition electrode are located on being determined by said reactance as being at least periodically smaller than the ignition current of the control gap at a certain predetermined irradiation of the photosensitive electrode, but at least periodically greater than the ignition current of the control gap at a certain lower intensity of said irradiation, conditions of the discharge in the control gap being such that the discharge current becomes unstable and a surge discharge occurs, the current intensit of which at least periodically exceeds that of the preliminary current, when the ignition current is smaller than the preliminary current owing to a, decrease in irradiation, said surge discharge causing the ignition of the mentioned main gap.

2. A device in accordance with claim 1, characterized in that for increasing the ignition vol"- age of the main discharge gap insulating parts are disposed in the discharge vessel.

3. A device in accordance with claim 1, char-- acterized in that an ionisation chamber determining the preliminary current and constituting the reactor is provided.

4. A device in accordance with claim 1, characterized in that a pre-exposed photo-electric cell determining the preliminary current and constituting the reactor is provided.

5. A device in accordance with claim 1, characterized in that the reactor determining the preliminary current is located at least partly inside the discharge vessel.

6. A device in accordance with claim 1, characterized in that at least one electrode is of per forated or sievelike design.

7. An electric gas discharge tube for indicating a charge of light, said tube comprising a casing, two electrodes located within said casing and forming a discharge gap, one of said electrodes being photo-sensitive and being irradiated, a third electrode located within said casing and forming another discharge gap with said photosensitive electrode, and means connected with said photo-sensitive electrode and said third electrode and including a reactance for passing a weak electrical preliminary current through the first-mentioned discharge gap, the magnitude of said preliminary current being determined by said reactance as being such that it is at least periodically smaller than the ignition current at a certain predetermined intensity of said irradiation but at least periodically greater than the ignition current at a certain predetermined lower intensity of said irradiation, conditions of the gas discharge being such that the discharge ourrent becomes unstable and a surge discharge, the current intensity of which at least periodically exceeds that of the preliminary current, occurs When the ignition current is smaller than the preliminary current owing to a decrease in irradiation, said surge discharge causing the ignition of the second-mentioned discharge gap.

8. A device in accordance with claim 7, characterized in that a surge capacitor is connected in parallel to the two electrodes of the ignition gap.

9. A device in accordance with claim 7, characterized in that at least one electrode of the surge capacitor is so disposed as to exclude the outer air.

10. .A device in accordance with claim 7, characterized in that the two electrodes of the gap have surfaces of differing area.

11. A device in accordance with claim 7, characterized in that a glow tube determining the preliminary current and constituting the reactor is provided. 7

12. A device in accordance with claim 7, characterized in that the reactor takes the form of a capacitor.

13. A device in accordance with claim 7, characterized in that the electrodes between which the secondary discharge takes place have surfaces of differing area.

14. A device in accordance with claim 7, char" acterized in that at least one fourth electrode is provided and in that the main current of the secondary discharge flows through two electrodes different from the two electrodes of the ignition gap.

ERNST MEILI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,800,337 Campbell Apr. 14, 1931 1,840,055 Rentschler Jan. 5, 1932 1,973,286 Knowles Sept. 11, 1934 1,980,198 Gray Nov. 13, 1934 1,996,556 Sinden Apr. 2, 1935 2,098,217 Aubert et al Nov. 9, 1937 FOREIGN PATENTS Number Country Date 337,477 Great Britain Nov. 6, 1930 

